1. Field of the Invention
The present invention relates to a film forming apparatus and a film forming method used to manufacture an EL element composed of an anode, a cathode and a light emitting material, especially a self-light emitting material, for providing EL (electro luminescence) (hereinafter referred to as EL material), with the EL material sandwiched between the anode and the cathode. The EL material herein refers to a material that provides fluorescence or phosphorescence when an electric field.
In the present invention, a light emitting device refers to an image display device, or a light emitting device, that uses an EL element. Also, the following modules are all included in the definition of the light emitting device: a module obtained by attaching to an EL element a connector such as an anisotropic conductive film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of a TAB tape or a TCP; and a module in which an IC (integrated circuit) is directly mounted with a light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
In recent years, a technique of forming a semiconductor element on a substrate has greatly advanced and application of the semiconductor element to active matrix display devices (light emitting devices) is being developed. The semiconductor element refers to a single element, or a plurality of elements, formed of a semiconductor material and having a switching function. Given as an example of the semiconductor element are transistors, in particular, field effect transistors, typical example of which are a MOS (metal oxide semiconductor) transistor and a thin film transistor (TFT). A TFT formed of a polysilicon film can operate at high speed since the TFT is high in field effect mobility (also called mobility) compared with a conventional TFT that is formed of an amorphous silicon film. This makes it possible to control pixels by a driving circuit formed on the same substrate as the pixels instead of using a driver circuit outside the substrate as in the past.
The active matrix display devices as above have various circuits and elements formed on the same substrate, whereby a diversity of advantages are obtained including reduction in manufacture cost, miniaturization of electro-optical devices, raised yield, and an increase in throughput.
On the other hand, the light emitting device that is being vigorously researched is an active matrix light emitting device which has an EL element as a self-light emitting element (also called an EL display).
In this specification, the EL element of the light emitting device has a structure in which an EL layer is sandwiched between a pair of electrodes (an anode and a cathode). The EL layer generally takes a laminate structure. A typical example of the laminate structure is the one proposed by Tang et al. of Eastman Kodak Company, and consists of a hole transporting layer, a light emitting layer and an electron transporting layer. This structure has so high a light emission efficiency that it is employed in almost all of light emitting devices that are under development at present.
Other examples of the laminate structure include a structure consisting of a hole injection layer, a hole transporting layer, a light emitting layer, and an electron transporting layer which are layered in this order on an anode, and a structure consisting of a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injection layer which are layered in this order on an anode. The light emitting layer may be doped with a fluorescent pigment or the like.
In this specification, all of the layers provided between a cathode and an anode are collectively called an EL layer. Accordingly, the hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injection layer, etc. mentioned above are all included in the EL layer.
A predetermined voltage is applied to the EL layer with the above structure from the pair of electrodes, whereby recombination of carriers takes place in the light emitting layer to emit light. The EL element in this specification refers to a light emitting element composed of an anode, an EL layer, and a cathode.
The EL layer of the EL element is degraded by heat, light, moisture, oxygen, etc. Therefore, the EL element is generally formed after wirings and TFTs are formed in a pixel portion in manufacturing an active matrix light emitting device.
The EL layer described above can be formed by various methods. Examples of the methods that have been proposed include vacuum evaporation, sputtering, spin coating, roll coating, casting, the LB method, ion plating, dipping, the ink jet method, and printing. The printing is a particularly effective method because the EL layer can be formed selectively.
After the EL element is formed, the substrate over which the EL element is formed (EL panel) is bonded to a covering member by sealing with a sealing member or the like (packaging) without exposing the EL element to the outside air
After the packaging or other processing for enhancing airtightness, a connector (FPC, TAB, or the like) is attached in order to connect an external signal terminal to a terminal lead out of an element or a circuit formed on the substrate. The active matrix light emitting device is thus completed.
When printing is employed to form the EL layer, a print material changes with time if a solvent for dissolving an EL material is highly volatile. This makes it difficult to process a large number of substrates.